1. Field of the Invention
The present invention relates to a voltage regulator for outputting a constant voltage, and more particularly, to a reduction in power consumption of a voltage regulator.
2. Description of the Related Art
A voltage regulator is aimed to supply a stable voltage to an electronic device connected to an output, irrespective of fluctuations of an input voltage or output current supplied to a load. The voltage regulator has a wide range of use, and is used for stably operating information devices, portable communication devices, and the like.
In the portable communication devices, to achieve downsizing and lightening of batteries, thereby prolonging the operation time, is a top priority from the viewpoint of nature of the device. For combined attainment of securing a long operation time and downsizing and lightening of the batteries, reduction of power consumption of the device including the voltage regulator is effective.
The power consumption Pd of the voltage regulator is expressed by the following formula (1).Pd=Vin·Iss+(Vin−Vout)·Iout  (1)
In the formula (1), Vin represents an input voltage into the voltage regulator, Vout represents an output voltage from the voltage regulator, Iout represents an output current supplied from the voltage regulator to a device connected to a load, and Iss represents current consumption that is necessary for operating the voltage regulator itself.
In this case, Vout and Iout are determined based on specifications required for a circuit connected as a load of the voltage regulator. Therefore, in order to reduce the power consumption of the voltage regulator, it is necessary to reduce Vin−Vout, namely, the input/output voltage difference, and to reduce Iss, namely, current consumption of the voltage regulator.
In a voltage regulator having a small input/output voltage difference, which is referred to as a low drop-out (LDO) voltage regulator, a PMOS transistor suitable for reducing the difference between the input voltage and the output voltage is used as an output driver. In this case, the smallest input/output voltage difference which is necessary for operation of LDO is substantially proportional to an on-resistance of the output voltage. Accordingly, in order to reduce the input/output difference in the same process, a W length of the output driver has to be made larger, which means an increase in an area of a gate.
On the other hand, the voltage regulator controls the output driver so that a reference voltage therein and a reference voltage for monitoring a voltage to be output by the voltage regulator are made equal to each other. To reduce fluctuations of the output voltage at a transient response time, such as an abrupt change of a load current, is determined depending on how soon a gate potential, which is a control terminal of the output driver, may be changed. The gate terminal of the output driver has a large parasitic capacitance. Therefore, in order to quickly change the gate potential, there is no way but making an operating current of a differential amplifier circuit larger, which serves as a charge/discharge current for the gate, or making a value of a gate capacitance smaller by reducing a gate area. This indicates the existence of a trade off between the input/output voltage difference and the current consumption, which leads designing of a voltage regulator having small power consumption to difficult.
As a structure in which current consumption is suppressed and transient response characteristics are improved, there is proposed a circuit as illustrated in FIG. 2.
A conventional voltage regulator illustrated in FIG. 2 monitors an output current with a transistor 6 connected in parallel with an output transistor 9, and feeds back a current proportional to the output current to a tail current of a transistor 8, namely, a differential amplifier circuit. With this circuit structure, an operating current of the differential amplifier circuit increases in proportional to an output current of the voltage regulator. Accordingly, it is possible to improve the transient response characteristics under heavy load while suppressing current consumption of the voltage regulator under light load.
Further, as a technique of reducing power consumption other than the technique described above, it is effective in reducing power consumption to provide two states including a normal operation state in which the voltage regulator itself is subjected to a regulation operation of the output voltage and a standby operation state in which the regulation operation is stopped to reduce the current consumption of the voltage regulator itself.
However, in the conventional voltage regulator having the structure illustrated in FIG. 2, aside from the feedback system for a normal output voltage signal, there exists a feedback system for feeding back the output current to a differential amplifier circuit. Therefore, in a case where operating points of both the systems are simultaneously moved, the operation may become unstable due to interaction of the respective feedback systems.